1. Field of the Invention
The present invention relates to a radiation image sensing apparatus using X-rays or the like, which is used for radiologic equipment or the like and an image sensing method for the radiation image sensing apparatus and, more particularly, to a controller for performing emission stop control on radiation such as X-rays.
2. Related Background Art
As an X-ray image sensing apparatus, the present applicant has proposed an image sensing apparatus and method which perform exposure control by using a phototimer in, for example, Japanese Laid-Open Patent Application No. 10-327317. FIG. 9 is a block diagram showing the image sensing apparatus disclosed in this reference. Referring to FIG. 9, this apparatus includes an X-ray source 101 for emitting X-rays in the form of pulses and an X-ray image sensing panel 102. The X-ray image sensing panel 102 is comprised of a plurality of photoelectric conversion elements arranged two-dimensionally and a drive circuit of the elements. The X-ray image sensing panel 102 is driven by a panel drive circuit 103.
A phototimer unit 105 is arranged between the X-ray image sensing panel 102 and an object 104. The phototimer unit 105 is a sensor for detecting X-rays transmitted through a reference part (e.g., alveolar part) of the object 104 during image sensing exposure. Outputs from the phototimer unit 105 are integrated by an integrating circuit 106. The resultant value is output to a comparator 107. The comparator 107 compares this integration output with a reference value Vth, and outputs the comparison result to an X-ray source drive circuit 108 for driving the X-ray source 101. The X-ray source drive circuit 108 is controlled by an output signal from the comparator 107. When the output from the integrating circuit 106 exceeds the reference value Vth, the X-ray source drive circuit 108 stops driving the X-ray source 101 to stop X-ray emission.
This apparatus also includes a sensing start switch 109 for designating the start of image sensing of the object 104, an A/D converter 110 for A/D-converting a signal from the X-ray image sensing panel 102, an image processing circuit 111 for processing an image signal from the A/D converter 110, a monitor 112 for displaying a sensed image, and a recording medium 113 for recording sensed image data.
FIG. 10 is a timing chart showing the operation of the image sensing apparatus in FIG. 9. When the sensing start switch 109 is turned on, a signal for designating the start of image sensing operation is supplied from the sensing start switch 109 to the panel drive circuit 103, integrating circuit 106, and X-ray source drive circuit 108 (the sensing start switch in FIG. 10). Upon reception of this signal from the sensing start switch 109, the X-ray source drive circuit 108 starts driving the X-ray source 101 to emit X-rays from the X-ray source 101 (X-ray output in FIG. 10). Upon reception of the signal from the sensing start switch 109, the panel drive circuit 103 starts driving the X-ray image sensing panel 102 (X-ray image sensing panel operation in FIG. 10). The integrating circuit 106 resets an output from the phototimer unit 105 and starts integration (integrating circuit output in FIG. 10).
The X-rays emitted from the X-ray source 101 are transmitted through the object 104 which is a patient to be diagnosed. At this time, the X-rays transmitted through the object 104 vary in transmission amount depending on the sizes and shapes of bones and internal organs, the presence/absence of a focus, and the like in the object 104, and include image information about them. The X-rays transmitted through the phototimer unit 105 are converted into visible light by a phosphor (not shown). This light is incident on the X-ray image sensing panel 102. In the X-ray image sensing panel 102, sensed signals are accumulated by the photoelectric conversion elements arranged two-dimensionally (X-ray image sensing panel operation in FIG. 10).
The integrating circuit 106 integrates outputs from the phototimer unit 105. The output from the integrating circuit 106 gradually increases (integrating circuit output in FIG. 10). When the output from the integrating circuit 106 exceeds the reference value Vth, the comparator 107 outputs a signal for designating a driving stop to the X-ray source drive circuit 108, thereby stopping X-ray emission from the X-ray source 101 (X-ray output in FIG. 10). Thereafter, the image processing circuit 111 reads the sensed signal through the A/D converter 110 and performs predetermined image processing. The image processing circuit 111 also displays the sensed image on the monitor 112 or records the image data on the recording medium 113 (X-ray image sensing panel operation in FIG. 10).
In the above conventional X-ray image sensing apparatus, however, since interruption of X-rays is controlled by using the phototimer unit, the following problems arise. The phototimer is very expensive, and X-rays are slightly attenuated when they pass through the phototimer unit, resulting in a deterioration in S/N characteristics.
When the phototimer unit is used, a doctor or examination technician selects one or two of switches SW1 to SW3 in accordance with an image sensing position before image sensing to select a sensor for detecting X-rays, as shown in FIG. 11. When, for example, the lung of the object 104 is to be sensed, the two side sensors are selected, as shown in FIG. 12A. When the stomach is to be sensed, the central sensor is selected, as shown in FIG. 12B. In addition, the doctor or examination technician determines the reference value Vth for the integrating circuit 106 in accordance with an image sensing position before image sensing. When, for example, the lung of a person is to be sensed, since a high S/N ratio is required, the reference value Vth is set to be high. When the stomach is to be sensed, since high contrast can be obtained owing to barium, the reference value Vth is set to be low.
Selecting sensors and setting a reference value in accordance with an image sensing position lead to a deterioration in operability. In addition, if the image sensing position deviates from a proper position, the error between the X-ray amount at the actual image sensing position and the amount detected by the sensor increases. Optimal exposure cannot therefore be performed, resulting in a decrease in S/N ratio, an increase in X-ray dose, and a deterioration in image quality.